An internal combustion engine typically includes at least one cylinder that receives a piston to form a combustion chamber. The piston is connected to a crankshaft such that a rotation of the crankshaft results in a corresponding reciprocating motion of the piston within the cylinder. Intake and exhaust valves associated with each combustion chamber are indirectly connected to the same crankshaft by way of a cam arrangement such that a rotation of the crankshaft results in corresponding opening and closing movements of the intake and exhaust valves. The shape of cam lobes within the cam arrangement governs valve lift during actuation of the intake and exhaust valves. Thus, the movements of engine valves and the motion of the associated piston, because of their connections to a common crankshaft, are typically fixed and synchronized.
In some situations, however, it may be desirable to disconnect or otherwise modify the movement relationship between the engine valves and the piston. For example, the exhaust valves may be opened early to provide engine braking, or held open late to facilitate operation in the Miller Cycle. Similarly, open durations of the intake and/or exhaust valves may be caused to overlap, thereby implementing internal exhaust gas recirculation. In any of these situations, a variable valve actuator (VVA) is required to modify valve opening profiles away from the typical cyclical motion induced by the cam arrangement.
One common VVA includes a hydraulic piston situated to act on a rocker arm. The rocker arm is connected between the cam arrangement and the engine valves to pivot and, thereby, transmit the rotational motion of the cam lobes to the translational motion of the engine valves. At an appropriate time in the opening/closing cycle of an engine valve, the hydraulic piston is driven by fluid pressure to engage the rocker arm and disconnect the rocker arm from the associated cam lobe. As long as the hydraulic piston remains engaged with the rocker arm, the cam lobe has little affect, if any, on the motion of the valve. At a predetermined time, the hydraulic piston is withdrawn from the rocker arm to return the driving motion of the engine valve to the cam arrangement.
Although the described VVA may adequately adjust valve actuation, it may be problematic. Specifically, as the piston is driven to engage the rocker arm, the engaging end of the piston impacts the rocker arm with significant force and then slides along a surface of the rocker arm. The sliding is caused by differential paths between the two components. That is, the rocker arm motion is rotational, but the piston motion is purely translational. This difference in motion creates the differential surface slip at the interface between the piston and the rocker arm. The impact, combined with the slip, can result in failure of the piston and/or the rocker arm at the engagement site.
One attempt at minimizing wear at the interface of the piston and the rocker arm is described in U.S. Pat. No. 6,718,846 (the '846 patent) issued to Ellison on Apr. 13, 2004. Specifically, the '846 patent describes an apparatus for aligning a cylindrical actuator (i.e., piston) with a bearing member, the bearing member having a planar surface configured to bear against and slide relative to a pivoting rocker arm during translational movement of the cylindrical actuator. The cylindrical actuator has a spherical end received by a spherical opening of the bearing member. The bearing member is held in swiveling contact with the cylindrical actuator by the spherical end of the cylindrical actuator and by a spring retainer located on the actuator at a distance from the spherical end. A spring is located about the actuator, from the spring retainer to the bearing member, such that a spring force is placed on the bearing member to maintain alignment of the bearing member with an axis of the cylindrical actuator. During pivoting motion of the rocker arm, the bearing member slides along the rocker arm and spreads the bearing force transmitted from the cylindrical actuator to the rocker arm across the area of the bearing member. As the cylindrical actuator withdraws from the rocker arm, the bearing member is maintained in contact with the actuator and also withdraws from the rocker arm.
Although the apparatus of the '846 patent may help to minimize wear at the piston/rocker arm interface, it may still be problematic. Specifically, the bearing member of the '846 patent may be under constrained. As such, perpendicularity of the cylindrical actuator relative to the rocker arm may be difficult to maintain, resulting in wear of the spring retainer through side loading. In addition, it may be possible for the bearing member to wander on the rocker arm, resulting in excessive wear of the rocker arm.
The variable valve actuator of the present disclosure solves one or more of the problems set forth above.